This article was originally published in Fly RC’s April 2016 issue.
By Gary A. Ritchie
In the world of RC scale model airplane building, one of the trickiest problems is how to “hide” external pushrods and control horns inside wing surfaces. With tail control surfaces these devices can normally be concealed within the fuselage. But on wings, which are often very thin, the problem is far more difficult.
Take a look at Figures 1 and 2. In Figure 1, an RC model Nakajima Ki-84 fighter, the aileron control horn and pushrod protrude from the lower wing, spoiling the scale appearance of the airplane. The model Spitfire Mk VB (Figure 2) suffers from the same problem.
But look now at the underside of my P-40 Kitt yhawk (Figure 3) and my Ilyushin Il-2M3 Sturmovik (Figure 4). No pushrods, no control horns – the wings are whistle clean and very scale-like, while the control surfaces move from within as if by magic. So, how to do it?
I know of two different methods of achieving this. The first method is simple, elegant and inexpensive, but it offers only a limited amount of control throw. So it is oft en used on ailerons. The second way is more costly, but it provides as much control throw as you need and, as such, works well with flaps. Let’s begin with the first method.
MIKE MACFARLAND METHOD
This method was reported in RC-Sport Flyer magazine a few years ago by my colleague Mike MacFarland. For each control surface you will need a small, ? at servo such as a Hitec HS-82MG, a large DuBro EZ-connect and a 1/2 inch long section of 1/8-inch diameter brass tubing. You will also need to cut a 3 inch long section of 3/32-inch diameter wire.
The installation is shown in Figure 5. On the left side of the photo is the HS-82MG servo mounted sideways in a plywood- reinforced rib. The brass tube was slipped through a 1/8-inch hole that was drilled through the EZ- Connect and then the EZ-Connect and the brass tube assembly were mounted on the servo horn.
The aileron is on the right side of the photograph. The 3/32- inch diameter wire has been bent at a 90° angle about 1 inch from one end and glued securely to the aileron with its long end protruding through the leading edge (Figure 6). In Figure 7, the leading edge of the aileron is pressed flush against the trailing edge of the wing with the metal wire protruding through the hole in the trailing edge where it slides through the brass tubing mounted on the servo horn.
Here is how it works. When the servo horn moves upward it pushes the end of the metal wire upward, while it slides back and forth through the brass tube. This causes the aileron to move downward and vice versa. This elegant litt le device is inexpensive and simple to build. Its only drawback is that the range of motion is limited by the thickness of the wing. With an aileron, this is normally not a problem because in scale models the control throw on ailerons is typically not very great. With flaps however, where up to 45° of throw may be needed, this device may not be suitable.
ROTARY AILERON LINKAGE METHOD
I was made aware of this method many years ago by a friend and fellow modeler named Kelvin Ritchie (no relation). This device is o? ered by Walt Dimick of I.F.R. Machine Works (www.irfmachineworks. com). It consists of a nylon/metal coupler that screws securely to the servo spline shaft , a long bent hardened drive shaft and a ? at “precision pocket” (Figure 8) into which the end of the driveshaft slips. The servo with the rotary linkage is mounted on its side inside the wing (Figure 9), while the precision pocket is mounted inside the control surface (Figure 10). The ? nished installation, without aileron, can be seen in Figure 11.
When the servo is actuated the end of the driveshaft , which is inside the precision pocket, rolls from side to side causing the control surface to move up and down. The design of this device provides a great deal of control throw.
What I oft en do is to use the inexpensive MacFarland method to drive my ailerons and the pricier rotary aileron linkages for my flaps. I have used these two devices on several of my scale ships, including the P-40 and the Il-2M3 shown in my photos, as well as a Mosquito night fighter that I reported on in RC-Sport Flyer in December 2009 (Figure 13). I find both methods to be highly reliable and would encourage you to try them out.